Among such X-ray analyzing apparatuses, the energy dispersive X-ray microspectrometers (EDX) have been applied to analysis of various materials, because they enable us to analyze two-dimensional distribution of elements with high resolution by easy operation.
Besides the energy dispersive X-ray microspectrometers, the applications of the X-ray measurement apparatuses, e.g. X-ray telescopes, have been widening.
In these X-ray measurement apparatuses, X-ray detectors must be protected from the contamination by open air. Thus, an X-ray transparent window should be settled between an X-ray detector and open air. Especially, if the detector is a semiconductor detector, an X-ray window is indispensable for protecting the semiconductor detector from the contamination.
For these reasons, the need for good X-ray windows has been increasing. First, as a property of the material of good X-ray windows, high transparency for X-rays is required. Glass optics which are properly employed for visible light or ultraviolet light are of no use for X-rays, because the absorption of X-rays of glass is too large. Secondly high strength is required for X-ray windows. An X-ray window must be very thin in order to decrease the absorption of X-rays and visible lights. Thus, such material, strong enough even in the form of a very thin film, is required for the basic material of X-ray windows.
Conventional X-ray windows for energy dispersive X-ray microspectrometers have employed beryllium as the material of the film. In addition to EDXs, beryllium windows have been used as the X-ray windows for synchrotron orbital radiation. Beryllium is strong enough even in the form of a thin film. The absorption of X-rays is comparatively small, because the atomic weight of beryllium is small. However, even beryllium windows must be thicker than several tens of microns to ensure the mechanical strength as a window. Such thick beryllium windows exhibit strong absorption for the X-rays scattered from light atoms, e.g. nitrogen atoms. Thus, the kinds of detectable elements are restricted for the X-ray detector with a beryllium window.
If an X-ray detector were used without an X-ray window, the X-ray detector would be contaminated in all probability. Therefore, almost all X-ray detectors are unavoidably equipped with beryllium windows at the expense of the sensitivity for light elements. This is the present state of X-ray windows.
Diamond has extremely high Young's modulus. Thus, it is believed that a very thin diamond film can keep its own shape because of the high rigidity. Besides, diamond has low absorption coefficient for X-rays. Diamond has been deemed a promising material for X-ray windows. However, the difficulty of processing diamond has been preventing a diamond X-ray window from being put into practice. It is difficult to polish a bulk diamond monocrystal till a thin film. There had been no good method for growing diamond thin film on a non-diamond substrate until late.
However, recent developments of the chemical vapor deposition methods have enabled us to grow diamond films or quasi-diamond carbon films on a pertinent substrate. Such a probability to make X-ray windows with a diamond film becomes within our reach. An X-ray window having a diamond film would be able to keep its inherent shape, even if it was thinner than 1 .mu.m, because of the high Young's modulus. Thus, the X-ray windows having a diamond transparent film would enjoy the advantages that the absorption of X-rays by the transparent film would be able to be decreased by thinning the diamond transparent film.
However, X-ray windows are generally used under severe environment. In the case of the energy dispersive X-ray microspectrometer, there is a considerable difference of pressure between the front and the back of the X-ray window. The pressure difference makes the X-ray window press inward. In the case of X-ray cosmic telescopes, impulsive acceleration acts on X-ray windows. In these cases, high mechanical strength is required for the X-ray window. A diamond film thinner than a few micrometers cannot satisfy the requirement for strength.
On the contrary, a thick diamond film which has sufficient mechanical strength would not be desirable because of the large absorption of X-rays.
Then, an X-ray window having a diamond film reinforced by silicon crosspieces as shown in FIG. 3 was invented. EP 365,366 disclosed on Apr. 25, 1990 proposed this reinforced window.
In FIG. 3, an X-ray transparent film (1) is sustained by a silicon ring substrate (3), although the peripheral part of the substrate (3) is left unetched, the central part is partially etched. Thus the substrate is called a ring substrate. The unetched parts constitute reinforcing crosspieces (12). The crosspieces (12) are made from silicon, because they are originally parts of silicon substrate (3). A supporter frame (4) is glued to the periphery of the silicon ring substrate (3). There are the diamond X-ray transparent film and the silicon reinforcing crosspieces in the range through which X-rays are transmitted. X-rays must pass through the silicon crosspieces (12) as well as the diamond film (1). The X-rays will attenuate by the silicon crosspieces because silicon will easily absorb for X-rays. This X-ray window has the advantage of facile fabrication, because the crosspieces of silicon are made only by etching away parts of the center of the silicon substrate (3).
In addition, the inventors of the present invention had proposed another X-ray window having a diamond X-ray transparent film reinforced by the crosspieces which is fabricated by evaporation-coating nickel, chromium or other metals with high rigidity in a lattice structure, e.g. lengthwise and crosswise on the diamond film. This is Japanese Patent Application NO. 1-308174 filed on Nov. 28, 1989.
The inventors had proposed another X-ray window having a diamond film reinforced by silicon crosspieces in Japanese Patent Application NO. 1-308173 filed on Nov. 28, 1989.
The silicon crosspieces were made by bombarding boron ions lengthwise and crosswise on a silicon substrate, depositing diamond on the silicon substrate, and etching away the silicon substrate. Since the portions of the silicon substrate bombarded by boron ions are not etched, the residual boron-doped parts become the crosspieces.
The X-ray windows having silicon crosspieces made by etching selectively the central part of a silicon substrate on which a diamond film was grown have the following disadvantages.
Thermal expansion coefficient of silicon differs from that of diamond. As the growth of diamond by the vapor phase synthesis is done at considerable high temperature, strong thermal stress will be generated between the silicon crosspieces and the diamond film by the difference of thermal expansion coefficients, when the specimen is cooled down to room temperature. Therefore, the windows are likely to be broken or distorted, when they are fitted to X-ray measuring apparatuses.
Other X-ray windows having crosspieces made from rigid metals except silicon also have the same disadvantage. Besides the disadvantage, the use of non-silicon metals for the crosspieces will complicate the fabrication procession, which takes us a lot of time and money.
The purpose of this invention is to provide an X-ray window which excels in X-ray transmittance, mechanical strength and suppression of thermal stress.